Method of carbonizing metals and alloys



C. V. IREDELL Jan. 3, 1933.

Filed May 25, 1928 Patented Jan. 3, 1933 UNITED STATES PATENT OFFICECHARLES V. IREDELL, OF BLOOMFIELD, NEW JERSEY, ASSIGN 'QR TOWESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA METHOD OFCABBONIZING METALS AND ALLOYS Application filed May 25,

This invention relates to metallurgy and more particularly relates tomethods of coating metals and comprises essentially in a method offorming an adherent gas-free carbon coating upon metal surfaces.

Inthe production of electron discharge devices particularly of therectifier and os-. cillator types, the use of high plate and fgridvoltages result in pronounced heating 0 the refractory metals comprisingthese electrodes. In some types this heating effect is sufiicient toraise the temperature of these electrodes tobright redness. Under suchtemperature conditions these electrodes are then capable of themselvesemitting electrons which disrupt the operating efliciency of thesedevices. One of the methods devised to overcome this heating effect isto cover the metal sur face of these electrodes with a material WhlChWill more readily radiate and dissipate the heat energy of the devicethan the bright metallic surface heretofore employed. Black coatingshave been found to serve this purpose admirably, and many methods havebeen developed wherein black carbonaceous coatings have been applied tothese surfaces.

It has been found, however, that although such blackening by coatingwith carbonaceous material is effective in reducing and even eliminatingback emission due to incandescin g the metal electrodes, the majority ofsuch coatings have introduced an additional factor in that unless highlypurified carbon is employed therein the contained impurities are brokendown under the influence of the electron bombardment and the resultantliberated impurities either produce back emis- 40 sioneffects or arevolatilized and destroy the emissive qualities of the filament.

Such carbonaceous coatings have also been difficult to cause to adhereto the metal surfaces, and are usually hard to free of ad- 45 sorbed andabsorbed gaseous impurities.

One of the objects of this invention is to provide a method of coatingmetal surfaces with substantially pure carbon.

Another object of this invention is to provide a method whereby aclosely adherent 1928. Serial No. 280,432.

I ment, and which is coated with a closely ad herent heat radiatingcarbon layer, substantially integral with but only superficiallyattached thereto.

Other objects and advantages will be apparent as the method is morefully disclosed.

My invention substantially is comprised of two olperations, eachcontaining two steps.

1. eoxidizing and carbonizing.

2. Degasifying and surface cementing.

The carbonizing step in the first operation is dependent upon obtainingan oxide-free metal surface to deposit the carbon upon. I have alsofound that thesubsequent degasifying and surface cementing step isdependent upon this procedure as it is essential that these metal partsbe free of oxygen carrying compounds.

This surface deoxidizing is obtained by means of heating the metal bodyin a reducinggas' (hydrogen) prior to carbonizing, thereafter the metalsurface is carbonized by introducing into the reducing gas a proportionof a hydrocarbon of the paraflin series and heating the deoxidized metalto a temperature sufiicient to effect decomposition or dissociation ofthe hydrocarbon vapor. The two steps in this operation are made in acontinuous or consecutive manner without intervening exposure to anyother gases. The deposited carbon thereupon is formed in a densecoherent layer on the surface of the clean metal body. Upon cooling toroom temperature this coating is treated by the second operation of myprocess to effect consolidation and firm cementationof the carbon uponthe surface of the metal body.

In the second operation the coated surface 1 and the metal -body aremutually degasifie'd and cemented together by a second heating in y ahigh vacuo for a prolonged time interval. The temperature of heating isnot suflicient to cause any more than superficial formation of metalliccarbides but provides a means whereby these carbonized metals may belatterly exposed to atmospheric contamination without the formation ofsurface oxides which are decomposable under the influence of electronicbombardment, and normally inherent on most metal surfaces. The removalof this oxide film is essential in order to prevent the destruction ofthe activating constituent of the hot cathode in an electron-emittingdevice by interaction with the liberated oxygen of the electronicallydecomposed surface oxides.

In this manner I obtain, by the practice of m invention, a carbon coatedmetal body suita is for use in electron discharge devices which has acoating that is not only adherent sufiiciently to prevent the formationof oxide films thereon but of a sufficient thickness to substantiallyserve as a heat radiating surface, and moreover is of a degree of puritysuch that the hack emission of electrons due to the unavoidable heatingeffect produced during the operation of the electron discharge device isrestrained and in most cases inhibited.

In the practice of my invention 1 preferably employ the formed andfinished plate, grid and support material, although I have found thatthe coating I obtain by my process is smliciently adherent whenappiiedto sheets so that the plate or anodes may be stamped or formed therefromsubsequert to the carbonizing step. Thestamped or formed electrodes maythen be put through a degasifying or vacuum treating operation asdescribed above.

In the practice of my invention 1 have employed various members of thehydrocarbons of the parafiin series, some of which are gaseous at roomtemperatures, such as methane, and others of which have higher boilingpoints but high vapor pressures and which may be carried over the heatedmetal surface by saturating an inert or reducing gas with the vapor ofthe hydrocarbon liquid at room temperatures.

The particular hydrocarbon of the parafdn series which i prefer toemploy in the carbonizing step is commonly known as petroleum ether,which may be purchased on the market.

Petroleum other is a composite hydrocarbon having a high vapor pressureand a boiling point ranging from 30 C. to -C., and may be introducedinto the carbonizing process as a Vapor by bubbling the hydrogen throughthe liquid petroleum ether in a manner shown and described in myspecific description.

I have determined that the paratfin series a very pure carbon as adecomposition prod-' not. With the refractory metal bodies commonlyemployed in the forming of anodes, grids and grid and plate supports inelectron discharge devices, such as iron, nickel, or refractory alloysof these metals with copper, chromium, manganese, etc., the carbondeposited at this temperature forms a close ly adherent film, which filmis very diflicult to remove even by abrasion methods, particularly whenthe metal body has been previously freed of the surface adherent oxidelayer and which film may subsequently be caused to uniformly adhere tothe surface and prevent the formation of surface oxide compounds bysuitable treatment in vacuo. As a specific embodiment of my process Iwill describe the procedure emploved in coating nickel with an adherentgas-free carbon layer.

In the accompanying drawing, Fig. 1 is a cross sectional schematicdiagram of the apparatus employed in the first or deoxidizing andcarbonizing step of myprocess; and,

Fig. 2 is a cross sectional schematic diagram of the apparatus employedin the second or degasification and cementation step of my process.

In Fig. 1, the carbonizing tube 1 which may be silica. iron, or any heatresistant al-.

loy is mounted in a furnace 2 which may be heated electrically or by gasas desired. Suitable connections such as 3 should be provided forpassing the carbonizing gas through the carhonizing tube 1, and wherethere is considerable length to the tube 1, suitable piping 4 forreversing the gas flow, I prefer to provide a shunting means, such asindicated at 5, of passing the hydrogen through the petroleum etherwithout interrupting the flow of gas through the carbonizing chamber andcarrying the resulting gas mixture vto the carbonizing tube 1. I alsoprovide a method,such as indicated at 6. of effecting a refilling of thechamber 7 with petroleum ether as the supply therein becomes exhausted.Suitable valves 8 are provided where indicated to provide a way ofcontrolling the process. In Fig. 2 the evacuating chamber give a smallflame at the gas outlets 9 at the opposite ends of the carbonizingchamber 1. When the air has thoroughly been displaced. thechamber 1 maybe heated to the carbonizing temperature. Furnace 2 should i Thehydrogen is thenpassed gen is diverted through the petroleum ether byclosing valve 8a and opening valves 8b and 8a, and the carbonizingcontinued within the indicated temperature range for a period of timesuflicient to effect complete surface coating. By regulating the gasflow with respect to the diameter of the carbonizing tube, length ofheating zone and surface area to be carbonized a uniform time intervalmay be established.

' In practice I have observed the following ichedules in the carbonizmgof nickel suraces Under these conditions the time interval isapproximately one hour.

I have found in the practice of this carbonizing method that thedirection of the gas flow through the carbonizing chamber should bereversed at frequent intervals. prefer to reverse the gas flow at 10minute intervals, in order to equalize, if possible, the carbonizingefl'e'ct at both ends of the carbonizing chamber.

At the conclusion of the carbonizing process the furnace is turned offand the chamber and contents allowed to cool to temperatures below 300(3., periodically reversing the hydrocarbon saturated gas ow until afterthe temperature has been lowered to 500 C. Thereafter until cooled toroom temperature the hydrocarbon gas may be shut off and an atmosphereof pure hydrogen passed through the chamber.

The cooled carbonized metal parts are then cleaned of the excess looselyadherent carbon by brushing or by tumbling in sawdust, and are ready forthe second step .in my process.

The second step comprises a high temperature vacuum treatment for thepurpose of removing adsorbed or absorbed gases and in the accompanyingdrawing (Fig. 2) is similar to that used in the carbonizing procedurewith the exception that instead of gas flowing through the chamber 1,the chamber is hermetically sealed and connected in any suitable mannerto an exhaust system as indicated, wherein a vacuum approximating 0.25mm. of mercury may be obtained within the chamber. Valve 8 is placed inbetween the evacuating means and the chamber to facilitate handling.

The carbonized metal parts, after cleaning off the excess carbondeposit, are placed in the chamber and heated by any suitable means tobetween 850 C. to 950 C. for from one to one and one-half hours or untila gas pressure of not more than 0.25 mm. has been maintained for aprolonged period of time. The heating of the metal parts during thedegasification process should proceed from room temperatures to themaximum temperature with regular increases in the temperature gradientwhile evacuating the liberated gases continuously as liberated.

The degasified carbonized parts should then be cooled to roomtemperature before exposure to the air and until used in theconstruction of electron discharge devices sure to air prior toincorporation within theelectron discharge device.

Nickel parts so treated and carbonized substantially as described willbe found to have thereon a closely adherent carbon coating whichmoreover is substantially free of electronically decomposableintermet-allic compounds which would deleteriously affect theoperatingefiiciency of the hot cathode of an electron discharge device,and moreover will be found to be substantially free from deleterious anddiflicultly evacuated gases. It is believed that the temperaturesemployed are not sufiicient to effect appreciable formation of metalliccarbide formation except in so far as is necessary to cause adherence ofthe deposited carbon to the surface of the metal body.

When these metal parts are incorporated within an electron dischargedevice they will be found to more efiiciently radiate the heat energythan heretofore obtained by other methods of applying the carbon coatingand.

thereby reducing and substantially eliminating that source ofback-emission trouble which the uncoated electrode material norto causethe complete'cementation of the demally would develop. In additionthereto,

posited carbon to the surface of the metal ody. The apparatus employedas indicated the high degree 5f purity obtained in the carbon depositsubstantially eliminates that source of back-emission which has beenencountered in prior processes from the electronically decomposablecompounds inadvertently included therein.

Having broadly outlined the scope of my invention and specificallydescribed the same as regards to nickel, it is obvious that manydepartures from the specific process may be made without departing fromthe nature of the same. By the process as described with nickel andwithin the temperature and times specified, I have been able tosuccessfully carbonize iron, various nickel alloys with manganese, iron,chromium, copper, etc. More refractory metals, such as molybdenum andtungsten would require substantially higher temperatures to produce thesame effect as is at present obtained with nickel, iron or the alloys ofthese metals with other metals.

Such variations are anticipated as should fall within the scope of thefollowing claims.

lVhat is claimed is:

1. The method of producing adherent carbon coatings on metal surfaceswhich comprises heating a metal first in an atmosphere of a gas reducingwith respect to the oxygen compounds of the metal thereafter introducinginto said gas atmosphere without intervening exposure of the metal toother gases a proportion of the vapor of a hydrocarbon of the paraflinseries, and continuing the heating of the metal body, the temperature ofheating approximately the decomposition temperature of the hydrocarbonvapor employed. I

2. The method of forming an adherent carbon coating on nickel surfacescomprising heating the nickel to approximately 800 C. in an atmosphereof hydrogen subsequently introducing into the hydro en atmosphere aproportion of the vapor oi a hydrocarbon of the paratlin series andcontinuing the heating until a suflicient depth of carbon coating hasbeen obtained.

' 3. The process of forming an adherent carbon coating on the surface ofrefractory metals, the oxides of which are reducible by hydrogen, whichcomprises heating a metal body to approximately 800 C. in an atmosphereof pure'dry hydrogen to effect reduction and removal of the oxygencompounds of the same, and thereafter introducing into the hydrogenatmosphere the vapor of a hydrocarbon of the paraflin series andcontinuing the heating at the same temperature until surfacecarbonization has become effected.

4:. The process of forming an adherent carbon coating on the surface ofanickel containing metal, which comprises heating a nickel containingmetal in an atmosphere of pure dry'jhydrogen to temperatures approximating 800 C. to effect substantial removal of the oxygen containingcompounds therein and thereon" and thereafter introducing into thehydrogen atmosphere the vapor of a hydrocarbon of the paraflin series,and continuing the heating at the same temperature until surfacecarbonization has become effected.

5. The process of forming an adherent carbon coating on a nickel surfacewhich com prises heating the metal body in an atmosphere of pure dryhydrogen to temperatures approximating800 C. to effect substantialremoval of the surface oxides and thereafter introducing into thehydrogen atmosphere the vapor of petroleum ether, andcontinuing theheating at the same temperature until surface carbonization has becomeeffected.

6. The process of treating electrodes for use in electron dischargedevices which comprises heating electrodes in a pure dry hydrogenatmosphere to effect reduction and removal of oxygen containingcompounds therein and thereon and thereafter carbonizing the cleanedmetal surface by heating the electrodes in an atmosphere containing aproportion of a hydrocarbon of the paraflin series to the decompositiontemperature of the same, without intervening exposure to atmosphericgases.

7. The process of treating nickel electrodes for use in electrondischarge devices which comprises heating nickel electrodes in a puredry hydrogen atmosphere to temperatures approximating 800 (1, andthereafter introducing into the hydrogen atmosphere a proportion of thevapor of a hydrocarbon of the parafiin series and continuing the heatingat approximately the same temperature for a suficient interval of timeto effect surface carbonizing of the nickel electrode.

8. The process of treating electrodes for use in electron dischargedevices which comprises deoxidizing the electrodes by heating in a puredry hydrogen atmosphere,-carb onizing the cleaned, surface by heating inavapor of a hydrocarbonof the paraffin series without interveningexposure to atmospheric gases and thereafter effecting cementation ofthe carbon coating to the metal base and substantial degasification ofthe carbonized electrodes by heating the same in a high vacuo prior toutilizing the electrodes as incorporated parts of arr-electron dischargedevice.

9. The process of treating nickel electrodes for use in electrondischarge devices which comprises deoxidizing the nickel electrodes byheating in pure dry hydrogen at temperatures 'a roximatin 800 C.carbonizin the PP z: a b

deoxidized surface by introducing into the hydrogen atmosphere aproportion of a vapor of a hydrocarbon of the parafiin series andcontinuing the heating without intervening exposure to atmosphericgases, and thereafter 'degasifying and cementing the carbon coating tothe metal surface of the electrodes by heating them to approximately 900C. in a vacuo of the order of .25 mm. of mercury prior to incorporatingthe electrodes into the electron discharge device.

.10. As an article of manufacture deoxidized nickel coated with asubstantially pure adherent carbon 'film. I

11. As an article of manufacture, deoxidized nickel coated with anadherent substan-' tially pure degasified carbon film.

12. As an article of manufacture, an electrode for electron dischargedevices composed of deoxidized nickel coated with an adherentsubstantially pure carbon film.

13. As an article of manufacture, an electrode for electron dischargedevices composed of deoxidized nickel coated with an adherent gas-freesubstantially pure carbon film.

In testimony whereoi, I have hereunto subscribed my name this 23rd dayof May 1928.

. CHARLES V. IREDELL.

